A typical electronic image sensor comprises a number of photodiodes or other photosensitive elements arranged in a two-dimensional array. These elements are also commonly referred to as picture elements or “pixels” and the corresponding array is referred to as a pixel array. Light incident on the pixel array is converted to electrical charge by the photosensitive elements. Collected electrical charge for a given image capture period is read from the photosensitive elements of the pixel array using an active pixel sensor (APS) or charge-coupled device (CCD) arrangement
In order to read out the pixel array at high speed, it is often necessary to share certain signal processing circuitry between multiple analog readout channels. Such multiple-channel readout arrangements generally involve multiplexing several low-speed analog channels into a single high-speed analog channel. For example, in a typical APS image sensor implemented using complementary metal-oxide-semiconductor (CMOS) circuitry, also referred to herein as a CMOS image sensor, the low-speed channels may correspond to respective columns of the pixel array, and each of the low-speed channels may include an analog signal processor (ASP) and a programmable gain amplifier (PGA). The high-speed analog channel may include a high-speed PGA or a sample-and-hold amplifier coupled to an analog-to-digital converter (ADC) which converts analog signals read from the pixel array into digital image data.
Unfortunately, significant problems can arise when multiplexing several low-speed analog channels into a single high-speed analog channel. For example, input switching operations associated with the high-speed PGA may disrupt the proper operation of the low-speed PGAs. Conventional analog multiplexers are unable to provide an adequate solution to these problems. Accordingly, a need exists for an improved analog multiplexing arrangement for use in a CMOS image sensor or other type of electronic image sensor in which multiple low-speed analog channels are multiplexed into a single high-speed analog channel in conjunction with image readout.